--NO SPOILERS--
Absolutely loved this film. Surprised at how many 1 star reviews there are. I see some people saying that "nothing happened" or that "there was barely any action", and to that I will say: if your definition of "Horror" involves a lot of gore or a lot of jump-scares then this movie is NOT for you. I feel like many negative reviews are from people who are emotionally desensitized and need a physical shock factor in order to feel scared.
I have never in my life been so on edge for a movie like I was for this one. Is it perfect? No. And looking back I do get the sense that a good chunk of the film (especially the "side story" as others have put it) was edited or cut. I would have liked to see more of Stephen Yeun's character but honestly I don't think you need to fully understand or even care about any of the characters for you to understand the horror of what they're going through. There was a good mix of REALLY tense moments and some funny moments where the characters say exactly what you're thinking about the situation. I do also agree that there was something missing at the end to really wrap things up, but overall it was a great film and I would love to re-visit it at home when it releases.
My advice: don't look for a deeper meaning or an over-arching "why" until after the film. I really feel that in order to get the most out of it, you should lean in to the guttural reactions that the film is meant to evoke. You can certainly draw some connections between it and the real world but it's not the "artsy-fartsy you need to be this smart to get it" kind of film that some negative reviews make it out to be.